+1 Programs
I can’t speak to the specific program, but in general, the +1 program’s aren’t too intense. You take some grad courses instead of undergraduate ones, and then take a year of grad classes. I wouldn’t worry about deciding now, as your curriculum won’t change in the first year based on it. Wait to make any decision on that.
Co-op and Summer Courses
Think of it this way: You usually need 8 semesters to graduate from any school. Northeastern is the same. Summer sessions = half a semester. So for every semester you miss, you need to do 2 summer sessions or an extra semester. If you do 2 co-ops in 5 years, you won’t need any summer classes. A +1 program will add a year to that just like anywhere else, no summers needed.
Don’t forget about degree credit differences and AP credit either. For example, a 5 year 3 co-op program usually includes 2 summer sessions, but with a semester’s worth of AP credit, you won’t need them.
Most people at Northeastern take at least one summer class session, and there are good offerings because of that. The campus is active pretty much 365 days a year minus winter break. They enable a lot of the 4 year programs with co-ops as well, though the most common by far is 5 years with 3 co-ops and 1-2 summer sessions depending on AP credit.
There’s a lot of nuances here, and everyone’s pattern is different. I’m actually doing something similar to you. I’m doing a BS/MS program with 2 co-ops. Because of AP credit and summer sessions (2 total), I’ll do it in 5 years and 9 total semesters, rather than the usual 10 semesters and 6 years.
Core/GenEd’s
The core curriculum isn’t 11 courses - it’s 11 learning goals. A single class can satisfy 2-3 of those at once. Some classes in your major will hit these goals. As long as you hit each one before you graduate, you’re good to go. Given co-op, every major (as far as I know) starts with classes in your major in your first year (usually 2/4 of your courses each semester). You can do them at any pace you like. That flexibility was a big draw for me, as well as diving straight into my major.